The present invention relates to a rare earth-based permanent magnet having a corrosion-resistant surface film and a method for the preparation thereof. More particularly, the invention relates to a permanent magnet based on neodymium, iron and boron and provided with a highly corrosion- and oxidation-resistant surface coating film.
As is known, permanent magnets of the composition based on neodymium, iron and boron as the principal constituent elements, hereinafter referred to as the Nd-Fe-B magnets, as a class of the rare earth-based permanent magnets have several advantages, as compared with conventional samarium- and cobalt-based permanent magnets, in respect of the high magnetic performance and absence of limitation in the availability of neodymium as one of the essential constituents. Therefore, the demand for such Nd-Fe-B magnets is rapidly growing along with the expansion in the application fields of such high-performance magnets including electric motors, actuators, sensors and the like, in particular, as electric parts in automobiles as one of the various application fields. A very serious drawback in these Nd-Fe-B magnets is that the corrosion resistance or oxidation resistance of the magnet, which can be a powder-metallurgically prepared sintered magnet or a so-called plastic magnet, is even worse than iron metal so that it is eagerly desired to develop a highly corrosion-resistant Nd-Fe-B magnet. Various attempts and proposals have been made but none of them can give satisfactory results.
Several methods have been proposed for the improvement of the corrosion resistance of the Nd-Fe-B magnets by the further addition of an adjuvant element to the magnetic composition [see, for example, Japanese Patent Kokai 59-64733 and 59-132104 and B. E. Higgind and H. Oesterreicher, IEEE Trans. Mag. MAG-23, 92 (1987)]. The adjuvant elements hitherto proposed include chromium, nickel, titanium and others, but addition of these elements, through very effective in improving the corrosion resistance of the magnet, is detrimental to the magnetic properties of the Nd-Fe-B magnet so that the amount of these adjuvant elements in the magnetic composition is limited to a very low amount and the advantageous improvements as desired by the addition thereof can hardly be obtained as a consequence.
Alternatively, it is proposed to provide the surface of a Nd-Fe-B magnet with a surface coating film of a material having corrosion resistance. For example, such a corrosion-resistant coating film is formed by electrolytic or electroless nickel plating, aluminum-ion chromating, spray coating of an epoxy resin, electrodeposition of an epoxy resin and the like [see, for example, Japanese Patent Kokai 60-63903, 60-54406, 60-63902 and 60-63901 and Papers in Research Meeting for Applied Magnetics, MSJ 58-9, 59 (1989)]. Each of these methods can be used in several particular applications and the technology in this regard has reached a stage where these methods are somehow practically applicable although not quite satisfactory results can be obtained in respect of the adhesion of the coating film to the substrate surface and the corrosion resistance obtained thereby, leaving problems for further improvements. It is known that, when a sintered Nd-Fe-B magnet is provided with a metal plating or resin coating, the corrosion resistance of the magnet obtained thereby greatly depends on the surface condition of the sintered body. For example, the corrosion resistance is decreased when the surface has an oxidized layer or working-degraded layer having poor magnetic properties or pores.